LONDON – There is only one view of the murder of the British soldier Lee Rigby on a south London street three weeks ago: horrific.

But there are two views of its significance. One is that it was an act by crazy people, motivated in this case by a perverted notion of Islam, but of no broader significance. Crazy people do crazy things, so don’t overreact. The other view is that the ideology that inspired the murder of Rigby is profoundly dangerous.

I am of the latter view. Of course, we shouldn’t overreact. We didn’t after the July 7, 2005, attacks on London’s public-transport system. But we did act. And we were right to do so. Our security services’ actions undoubtedly prevented other serious attacks. The “Prevent” program in local communities was sensible.

The government’s new measures seem reasonable and proportionate as well. But we are deluding ourselves if we believe that we can protect the United Kingdom simply by what we do at home. The ideology is out there. It is not diminishing.